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Date: Tue, 5 Jan 2010 07:34:02 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()
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To: Peter Zijlstra <peterz@infradead.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "minchan.kim@gmail.com" <minchan.kim@gmail.com>, cl@linux-foundation.org, "hugh.dickins" <hugh.dickins@tiscali.co.uk>, Nick Piggin <nickpiggin@yahoo.com.au>, Ingo Molnar <mingo@elte.hu>
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On Tue, 5 Jan 2010, Peter Zijlstra wrote:
> 
> If it were only unmount it would be rather easy to fix by putting that
> RCU synchronization in unmount, unmount does a lot of sync things
> anyway. But I suspect there's more cases where that non-busy matters
> (but I'd need to educate myself on filesystems/vfs to come up with any).

unmount may well be the only really huge piece.

The only other effects of delaying closing a file I can see are

 - the ETXTBUSY thing, but we don't need to delay _that_ part, so this may 
   be a non-issue.

 - the actual freeing of the data on disk (ie people may expect that the 
   last close really frees up the space on the filesystem). However, this 
   is _such_ a subtle semantic thing that maybe nobody cares.

It's perhaps worth noting that I think Nick's VFS scalability patches did 
at least _some_ of the "struct filp" freeing in RCU context too, so this 
whole "vfs delays things in RCU" is not a new thing.

But I think that in Nick's case it was stricly just the freeing of the 
inode/dentry data structure (because he needed to traverse the dentry list 
locklessly - he didn't then _use_ the results locklessly). So the actual 
filesystem operations didn't get deferred, and as a result it didn't have 
this particular semantic nightmare.

			Linus

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